Production of vitreous enamelled substrates

A method of providing a porcelain enamelled steel substrate which has at least one electrically-insulated aperture therein, said method comprising the sequential steps of

• (a) providing a steel substrate which includes at least one aperture that extends from one side thereof to the other and defines a longitudinal axis extending through said steel substrate,
• (b) plugging each aperture in said steel substrate with an electrically-insulated material,
• (c) applying a coating of a vitreous material to at least one surface of said steel substrate so as to cover said plugged apertures therein and provide a composite product,
• (d) forming a hole in the composite product obtained in step (c) along each said longitudinal axis, the diameter of said hole being less than the diameter of the aperture defining the longitudinal axis, and
• (e) firing said composite product obtained in step (d) to transform each coating of vitreous material into a porcelain enamelled layer and to thus provide an apertured porcelain enamelled steel substrate, each aperture having an electrically-insulating lining therein.